Pain is typically experienced when the free nerve endings of pain receptors are subject to mechanical, thermal, chemical or other noxious stimuli. These pain receptors can transmit signals along afferent neurons to the central nervous system and then to the brain. When a person feels pain, any one or more of a number of problems can be associated with this sensation, including but not limited to reduced function, reduced mobility, complication of sleep patterns, and decreased quality of life.
The causes of pain include but are not limited to inflammation, injury, disease, muscle stress, the onset of a neuropathic event or syndrome, and damage that can result from surgery or an adverse physical, chemical or thermal event or from infection by a biologic agent. When a tissue is damaged, a host of endogenous pain inducing substances, for example, bradykinin and histamine can be released from the injured tissue. The pain inducing substances can bind to receptors on the sensory nerve terminals and thereby initiate afferent pain signals. After activation of the primary sensory afferent neurons, the projection neurons may be activated. These neurons carry the signal via the spinothalamic tract to higher parts of the central nervous system.
One known class of pharmaceuticals to treat pain is the opioids. This class of compounds is well-recognized as being among the most effective type of drugs for controlling pain, such as post-operative pain. Unfortunately, because opioids are administered systemically, the associated side effects raise significant concerns, including disabling the patient, depressing the respiratory system, constipation, and psychoactive effects such as sedation and euphoria, thereby instituting a hurdle to recovery and regained mobility. Consequently, physicians typically limit the administration of opioids to within the first twenty-four hours post-surgery. Thus, it would be preferable to use non-narcotic drugs that deliver direct, localized pain control at a surgical site.
One drug class that is known to the medical profession is the alpha adrenergic receptor agonists. In general, the alpha-adrenergic receptors mediate excitatory and inhibitory functions: alpha-1 adrenergic receptors are typically excitatory post-synaptic receptors which generally mediate responses in the effector organ, while alpha-2 adrenergic receptors are located postsynaptically as well as presynaptically, where they inhibit release of neurotransmitters.
Examples of alpha adrenergic receptor agonists used clinically to treat different condition include clonidine, phenoxybenzamine and prazosin (for treatment of hypertension and opioid withdrawal), naphazoline (for nasal decongestion), UK-14,304 and p-aminoclonidine (for -glaucoma).
However, to date alpha adrenergic receptor agonists have not been widely appreciated as effective treatments for pain and/or inflammation. Thus, there is a need to develop alpha adrenergic receptor agonists to prevent, treat or reduce pain and/or inflammation.